An appealing feature of superconducting switching devices is practically zero conduction loss (resistance), which allows a scale-up of the device to a very high voltage and current without a penalty of added conduction loss. A widely used implementation of a superconducting switch is what is referred to as a fault current limiter that can be either resistive or inductive. In a resistive fault current limiter, the current passes through the superconductor and when a high fault current is created, the superconductor quenches. Specifically, the superconductor becomes a normal conductor and its resistance rises sharply and rather quickly. A resistive fault current limiter can be either DC or AC. If the fault current limiter is AC, then there will be a steady power dissipation from AC losses (i.e., superconducting hysteresis losses) resulting in rapid heating, which must be removed by the cryogenic system. An AC fault current limiter is usually made from wire wound non-inductively, otherwise the inductance of the device would create an extra constant power loss on the system. In contrast, an inductive fault current limiter is made from a transformer with a closed superconducting ring as the secondary. In un-faulted operation of the fault current limiter, there is no resistance in the secondary and so the inductance of the device is low. A fault current, however, quenches the superconductor. The secondary becomes resistive and the inductance of the entire device rises. The advantage of this design is that there is no heat ingress through current leads into the superconductor, and so the cryogenic power load may be lower. However, the large amount of iron needed in such devices means that inductive fault current limiters are much bigger and heavier than resistive fault current limiters.
The quench process in a superconductor is different between various types of superconductors. For example, the superconductor can be quenched by utilizing the non-linear property of the superconducting material, which rapidly becomes resistive when either one of the ambient factors, such as temperature (U.S. Pat. No. 4,803,456, incorporated herein by reference in its entirety), current, or magnetic field (U.S. Pat. No. 5,805,036, incorporated herein by reference in its entirety) exceed a certain critical value.
The easiest and the most reliable way to induce the transition is to heat the superconductor to a temperature above its superconducting critical temperature (TC). For example, high temperature superconductors (HTS), such as YBa2Cu3O7 (YBCO), quench when a small amount of non-superconducting current heats the material and raises it above 93 K. This can be accomplished by either by an external heater or by a radiant source, such as an infrared lamp (U.S. Pat. No. 6,472,966, incorporated herein by reference in its entirety). One known thermal superconducting switch includes a resistive wire wound about the superconducting wire. Normalization of the superconducting material in this system is effected by applying a DC current to the resistive wire, heating the superconducting material to above its critical temperature. In this conventional thermal superconducting switch, the resistive wire must be electrically insulated from the superconducting wire, for example by means of a layer of epoxy or by an insulating tape. Thermal conductivity of all common insulating materials falls with the temperature, therefore heat transfer at cryogenic conditions is usually slow and inefficient. However, because electrical insulating materials are also generally thermally insulating, poor thermal contact of the superconductor with the insulator results in a non-uniform heating, large thermal mass and slow operation of a thermal superconducting switch. The efficiency and speed with which the superconducting wire can be heated in these switches is, therefore, damped by the electrical insulator.
Moreover, the action of a superconducting switch, including the superconducting fault current limiter, depends on uniformity in the critical current of the tape. Since the superconducting tape is uniform at both micron scale and meter scale, heavy stabilization with the help of a stabilizer is required to prevent the superconductor from being damaged during the transition from the superconducting state to the normal state. The stabilizer adds thermal mass and makes the device inherently slow, especially during the recovery. Additionally a heavy copper stabilizer substantially reduces the “off” resistance of the device, thus limiting the role of the superconducting switch to that of the current limiter. As such, conventional thermally-switched superconducting switches with or without a stabilizer do not have sufficiently rapid switching times for many applications, such as SMES systems. Alternatively, radiant heating by means of lasers has been proposed (U.S. Pat. No. 3,956,727, incorporated herein by reference in its entirety). However because using lasers at cryogenic temperatures is impractical, they are rarely if ever used.
Therefore, there is still a continuing need to develop new fast superconducting switches that would allow for a rapid response time, and yet it would have a low-weight and remain stable.